Astronomical telescopes are becoming ever more popular with consumers as their prices have come down. Quality astronomical telescopes are now widely available at fairly reasonable costs. This has created a large number of inexperienced users of astronomical telescopes. Typical consumer-marketed refracting astronomical telescopes, when they are pointed at a celestial target, have the viewing end of their focusing tube positioned at an angle which would require a user to kneel or lie down and look up into an ocular lens positioned therein. It is common, therefore, for such telescopes to come equipped with a prismatic "diagonal" which redirects the image path 90 degrees so that a user can stand comfortably and look downward into an ocular lens.
The image presented through a refracting astronomical telescope with attached diagonal is reversed left to right. In order to correct the image for viewing, it is common to provide an elongate erecting lens between the ocular lens and the focusing tube. Furthermore, it is common to use an elongate barlow lens between the ocular lens and the focusing tube to multiply the magnification of the image provided by the telescope. When such elongate auxiliary lenses are used with a telescope, then a diagonal becomes absolutely necessary to avoid requiring a user to actually lie down to view the ocular lens. A problem with most diagonals is that they reverse the viewed image from right to left. For an inexperienced user, this presents a problem since spotting and tracking an astronomical object is non-intuitive. For example, when viewing the planet Jupiter, if an observer wanted to view the moons to the left of Jupiter, as the telescope is moved to the left, the viewed image appears to move to the right.
Another common practice with astronomical telescopes is the provision of multiple ocular lenses of differing focal lengths so that the effective magnification of the telescope is changed by changing ocular lenses. This presents a problem with manufacturers and retailers of such telescopes since the ocular lenses are small and can be easily removed and stolen from demonstration models or easily misplaced or lost by consumers. With advances in optical design and manufacturing, zoom lenses have become more affordable and are common in photography applications and terrestrial viewing devices such as binoculars. While a zoom lens with a refracting or catadioptric astronomical telescope would be a convenient and handy accessory, potentially replacing multiple ocular lenses, zoom lenses are not used with typical refracting or catadioptric telescopes due to the fact that compound lens systems such as are found in a zoom lens significantly reduce the amount of light reaching a user. Common diagonals also reduce the total light transmitted therethrough, so the light penalty associated with the combined diagonal and zoom lens is too great for practical viewing.
It is clear, then, that a need exists for an optical system which allows a user to conveniently view a refracting or catadioptric astronomical telescope from a standing position without affecting the left-right orientation of the image. Such a system should allow the reliable use of a zoom lens with a refracting or catadioptric astronomical telescope.